This invention relates to an improved automatic processing system and process for handling delicate objects in a controlled, low particulate atmosphere, and an improved method for constructing such apparatus. This system is particularly useful in the wet processing of semiconductor wafers, integrated circuits and similar delicate electronic devices.
In the manufacture of integrated circuits, there are many processing steps that involve the submersion in liquid processing media. This media may be a chemical media designed certain qualities to sections of the in process circuit, or it may have a cleaning function, or it may be a resist removal or it may be simply a wash step or the like. In all cases it is desired that both the atmosphere above the wet processing steps and the baths containing the fluid be as free of particulate matter as possible. The amount of particulates present in both the bath and the air will play a controlling role in the percentage of flaw-free circuits that can be produced in any given processing step. With only a slight reduction in particulate control procedures, the percentage of flaw-free circuits will fall to near zero.
Many improvements have been made to control the particulates during these delicate operations. A major improvement was the use of isolated areas, often called clean rooms, which have a controlled, carefully filtered, atmosphere. Care is taken in this room to control entry and exit and motion in the room. Entry and exit of personnel would be through an intermediate room or series of rooms that are successively cleaner. At each stage of entry, precautions are taken to prevent carry over of particles from the previous dirtier station. At the point of final entry, the entering person will normally have a complete covering of over garments which have a relatively low tendency to produce dust particles when moved, and are free from dust particles from the previous more dusty areas and cover the parts of the body such as skin, and hair, which can contribute dust particles. Breathing passages are usually covered with a filtering type mask.
Care is also taken within the clean room to prevent redistribution of dust particles that will have settled on static surfaces such as the floor, bench tops, or other work surfaces. From the standpoint of engineering controls, this is carried out by controlling the distribution of the incoming, filtered air. Precautions are generally taken, for example, to control the velocity of the air so that it is minimized to reduce its ability to lift deposited particles and render them air-born. Particularly efforts are made to prevent reaching the higher air velocities that will create turbulent flow patterns. Care is generally exercised to be certain that all incoming air is directed in a downward direction so as to minimize the tendency to lift the particles, causing them to again become airborne.
There is, however, a limit on the ability to control the air currents caused by the motion of a person in the clean room environment. Because of the physical size of the human body, and of the difficulty in controlling speed of movement, it is nearly impossible to move about in the clean room environment without creating turbulent air currents that will tend to lift settled particulates and cause them to be redistributed into the air where they have the opportunity to find a way to the in-process circuitry. One partial solution to this has been to reduce the amount of air movement caused by the human body by reducing the amount of the body entering the clean environment by the use of glove boxes. The glove box involves using large rubber gloves that are permanently attached to the wall of the room (which is normally significantly smaller then the walk-in type clean rooms). Hands and usually entire arms are carefully inserted into these gloves which can then be used to manipulate the integrated circuit precursors through various processing steps. Such a device would typically be called a glove box, or a clean glove box. In addition to the obvious size limitation imposed by the length of the human arm, this device has other limitations. The difficulty in controlling the speed and the erratic motion of the human arms still causes turbulent air flow with a resultant redistribution of settled dust particles. These difficulties along with difficulties performing repetitive operations with a high degree of reproducibility have made mechanical devices more popular.
When these mechanical devices are operated by computers, they overcome many of the above difficulties and have become very valuable in carrying out operations in clean rooms. They can be programmed to do repetitive tasks in very reproducible manners and they can be programmed to operate slowly enough to minimize the air turbulence created.
Such a device has been disclosed in U.S. Pat. No. 4,805,759. The '759 patent discloses "an installation for conveying delicate objects such as semiconductor layers during processing operations and for handling such objects in a controlled atmosphere, where, "wheeled carriages are drawn along tracks by an endless belt" and "By means of transfer tools, object-holding cassettes are taken from the carriages during operation and replaced on the carriages after processing". In the '759 patent, many of the problems associated with human operation in a clean room, as noted above, are overcome.
Unfortunately, the great number of sliding moving parts such as bearings and frictional surfaces utilized in the practice of the '759 patent cause tiny, dust-like, particles to be created as the metal wears. These particles then contribute greatly to the problems associated with attempting to maintain the reduced particulate atmosphere that is necessary in a clean room. The apparatus used in the '759 patent is also very large and cumbersome. Because of this, its movement creates more air movement than desired. This air movement in turn causes an undesired suspension and redistribution of the settled dust particles, which are unfortunately much more numerous because of the inherently dirty nature of this cumbersome machine. These dust particles cause devices such as integrated circuits produced by the practice of the '759 patent to have a lower percent of flaw-free, or minimum flaw, parts produced than desired.
In addition, the cumbersome nature of the '759 patent apparatus makes it difficult to maintain. To do repairs, it is necessary to enter the clean environment for an extended time, thus introducing contaminants when entering, and continuing to generate contaminants while in the clean area. The resulting contamination that is introduced to the clean room must be laboriously removed before the operation can be restarted. Even with the laborious cleaning procedures, there will often be a period of poor productivity after restarting the operation where the percentage of flawed parts will be higher than even the already high steady state percentage. The cumbersome nature of the apparatus with its many bearings, sliding surfaces and spread out nature, further, makes it more difficult to maintain.
In addition, the '759 patent uses conventional continuously rotating electric motors. The continuously rotating electric motors create additional problems that can only partially be dealt with. The requirement for a continuous flow of air for cooling, coupled with the frictional contacts (required for brush operated motors) makes necessary the isolation of the motor from the clean room. This isolation can be accomplished with a cowling as in the '759 patent, or by operating the motor outside of the clean room and bringing the shaft through the wall. Either of these techniques place severe mechanical restraints on design flexibility of the apparatus that will become clearer as our invention is defined further.
Further the apparatus of the '759 patent is relatively difficult to construct, with each part being different from other parts. With the '759 patent type of construction, it is necessary to have the entire area of the clean room (or clean box) set aside for the duration of the its assembly.
Another computer operated mechanical device for handling semiconductor wafers is disclosed in U.S. Pat. No. 4,904,153. The '153 patent discloses a "Transporting robot for semiconductor wafers". It describes a robotic arm assembly that is capable of moving items from processing station to processing station by traversing back and forth on an overhead rail within a clean room. The '153 patent suffers many of the difficulties of the '759 patent since its internal sliding rails provides for excessive particulate generation and the apparatus must be maintained by physically entering the clean room, with the resulting air currents that resuspend particulates. The resulting contamination and reduced productivity after repairs, results in a significantly higher rate of flawed integrated circuits than would be desired. Its non-modular nature makes construction and maintenance more expensive and difficult.
In the manufacture of integrated circuits, there are many wet processing steps that involve the submersion in liquid processing media. This media may be a chemical media designed to impart certain qualities to sections of the in process circuit, or it may have a cleaning function, or it may be a resist removal or it may be simply a wash step or the like. Typically employed in such wet processing steps would be etch solutions such as aqua regia (about 75% HCl and 25% nitric acid) often employed as a gold etchant, a buffered oxide etch such as ammonium fluoride and HCl in an aqueous base typically employed in a ratio of 4:1 to 20:1 as an etch for SiO.sub.2, a mixture of phosphoric acid, acetic acid, and nitric acid commonly employed at a ratio of 16:1:1 at 85% total conc. in water to etch metals, or nitric acid and hydrofluoric acid in a ratio of about 9:1, or mixtures of hydrochloric and nitric acid in a concentration of 37-38% and 71-72% frequently used with molybdenum, platinum or nichrome. Typically employed in such wet processing steps might be an organic solvent or mixture of solvents to remove one type of a photo resist, alternately an acid resist strip such as sulfuric acid and ammonium persulfate might be used for SiO.sub.2 resist stripping, or "phenolic stripper" solutions might be used for other types of resist and/or other substrates. In some cases chromic sulfuric acid resist removal baths might be used, particularly with metalized resist stripping. Resist developer solutions might also be used. Sometimes it is desired to used cleaning solutions such as the "two step process" for prediffusion cleaning of quartz ware where first water and hydrogen peroxide and ammonium fluoride in a ratio of 5;1;1 are used followed by a solution of water, hydrogen peroxide and hydrochloric acid in a 7:2:1 ratio, both at 75-85 degrees C., are used. Sometimes a rinse with a very clean organic solvent or water is desired.